FIELD OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART
This invention relates to a process for gasifying carbonaceous feed materials, particularly coal, and it particularly relates to a process for gasifying both caking and non-caking coals to produce a low or medium BTU fuel gas which is substantially free of condensible tars and oils.
One of the main sources of atmospheric pollutants today is from coal-fired electrical utility boilers. In these installations, a clean fossil fuel, such as natural gas, is not a practical substitute for coal in the generation of electricity because of scarcity and cost. Furthermore, the available supply of clean fuel may combat pollution more effectively when used to fulfill residential and small commercial needs.
As an example, combustion products of coal contribute approximately one-eighth of the total atmospheric pollutants emitted in the United States, including approximately one-half of the sulfur oxides and approximately one-quarter of both the nitrogen oxides andd of the particulates. Sulfur emissions from coal combustion may be reduced (a) by using low sulfur coal, (b) by cleaning high sulfur coal by physical methods, (c) by removing sulfur oxides from coal combustion gases, (d) by removing sulfur during the combustion step, (e) by producing de-ashed low sulfur fuel by the solvent processing of coal, and (f) by gasifying coal and removing sulfur from the gas before combustion.
The last procedure, gasification with gas cleaning before combustion, appears to offer the greatest reduction in sulfur emission since most of the sulfur and gasified coal appears as hydrogen sulfide. The removal of the hydrogen sulfide presents no great problem, however, since several different commercial gas cleaning processes are available today which can reduce the hydrogen sulfide content of gas streams from coal gasification to less than 10 PPM, and some processes remove hydrogen sulfide to 1 PPM or less.
Among the processes known for the conversion of coal to a fuel gas is that shown in Williams U.S. Pat. No. 2,805,189. In this process, coal is pretreated in a fluid bed prior to gasification at a temperature below about 600.degree. F. The resultant off gases are not recovered and a special solids transfer line is provided to insure that these gases do not enter the gasification reactor. This procedure results in a loss of some valatile hydrocarbons, lowers overall process yield, and present an additional gas stream that must be purified before disposal.
In Howard U.S. Pat. No. 2,582,712, particulate coal is preheated at 900.degree.-1400.degree. F in admixture with a large volume of residue from a gasification reaction zone. Specifically, about 15-30 units of hot residue from the gasification reaction are admixed with a single unit of fresh feed coal to rapidly heat the fresh feed to a non-caking temperature. This process requires the circulation of large amounts of residue thereby increasing the cost of the unit and subjecting the unit to considerable wear due to the abrasive nature of the residue.
In the Kalbach U.S. Pat. No. 2,687,950, there is disclosed a process for gasifying materials such as coal, including anthracite, bituminous and lignite, to produce a gas rich in hydrogen and a fuel gas having a relatively high heating value. The coal is first introduced into a carbonization zone for direct contact with hot gases at a temperature range of 1000.degree. to 1500.degree. F. The carbonaceous residue is introduced to a gasification zone.
The Matthews U.S. Pat. No. 3,884,649, shows a process wherein caking coal is converted to a fuel gas by first pretreating the caking coal and then passing the pretreated coal to gasification zone. In the Matthews invention, the off gases formed in the pretreater are passed to the gasification zone. The present invention is considered an improvement over the invention shown in the Matthews patent.